Fluorinated fluxless soldering

ABSTRACT

A method for pretreating a solder surface for fluxless soldering is disclosed. The method uses a noble fluorine gas to remove surface oxides from solder surfaces, without the use of external stimulation. A noble fluorine gas is suffused across the solder surface to reduce or eliminate or chemically convert the surface oxides. The process can take place at atmospheric pressure and room temperature. A simple belt driven transport may be used to move the parts past a nozzle which emits the vapor in a system similar to a conventional solder reflow machine.

FIELD OF THE INVENTION

This invention relates to soldering methods and systems, and moreparticularly to fluxless soldering in a fluorine enriched environment toreduce surface oxides.

INCORPORATION OF RELATED DISCLOSURES

This specification incorporates by reference U.S. Pat. No. 4,921,157issued on May 1, 1990, application Ser. No. 08/155,020 filed Nov. 19,1993, now U.S. PAt. No. 5,407,121; and application Ser. No. 08/339,770filed Nov. 15, 1994 as a division of application Ser. No. 08/155,020,now U.S. Pat. No. 5,499,754, all assigned to MCNC, Research TrianglePark, N.C.

BACKGROUND OF THE INVENTION

The ability to assemble an electronic circuit from various discretecomponents or integrated circuit chips requires that they be properlyconnected for electrical conductivity and fixed into position for longterm mechanical integrity. Absent such good, long term mechanicalstability and electrical conductivity the reliability of any circuit isseriously degraded.

Current fabrication techniques involve the use of electronic packagingsubstrates, such as printed circuit wiring boards that electricallyconnect and mechanically support various electrical elements, includingdiscrete components (such as resistors, transistors, diodes, switchingarrays, etc.) integrated circuit components (such as memory devices,microprocessors, multiplexers, etc.), and various other circuit elements(such as transformers, connectors, heat sinks, etc.). The contactsbetween the printed circuit board and the electrical device must havephysical, chemical, electrical and mechanical integrity and stability.

Soldering is the primary process used for connecting the electricalelements to the printed circuit board and fixing them in position.Soldering involves the use of a low melting point metal alloy, usuallyof the lead-tin type, that will join metals together at temperaturesaround 450° F. In one technique, metal pads are formed on the exposedsurface of the printed circuit board and receive a coating of solder, ora small solder bump. The electrically conducting leads on the circuitcomponent are brought into contact with the metal pads, heat is appliedto raise the temperature of the pads and leads, and the solder is thenheated to reflow and join the leads and pads. When the solder cools itresolidifies, thereby providing mechanical strength and a unifiedelectrical connection. However, most metallic surfaces, including thesolder, metal pads and leads, will oxidize in normal use, hindering thesoldering process or even causing it to fail.

Most soldering processes include three basic steps: (1) pre-cleaning anddeoxidation of the solder surface; (2) solder reflow and/or reflowjoining; and (3) post-soldering cleaning. The solder reflow is critical.The reheated solder must flow freely between the circuit board pad andelectrical lead to provide an air tight seal and a strong mechanicalbond. However, proper reflow can occur only after any oxide coating isremoved from the surface to be soldered because the high melting pointoxides will interfere with or even prevent wetting of the two surfacesto be joined.

Pre-cleaning is conducted to remove impurities and oxide from thesurface of the metal and solder that would create electrical resistance,weaken mechanical stability, or cause long term degradation of theunion. Different flux materials are used in the pre-cleaning to preparethe surface and facilitate an unimpeded reflow of the liquid solder toperform an air tight and conductive bond between the pad and electricallead. Activated fluxes, such as zinc, ammonium chloride, mineralacid-containing materials, and the like, are typically used in "coarse"soldering applications, i.e., repairing coarse wiring in motors orhouses. Highly acidic fluxes are used for soldering aluminum layers.Aluminum has a tenacious oxide layer which is chemically very inert anddifficult to remove. Fluxes used with aluminum can contain metalchlorides, fluorides, and ammonium compounds.

Unfortunately, the corrosive nature of the fluxes is becomingincreasingly incompatible with the sensitive microelectronic assemblies.For most microelectronic devices, the standard practice is to reduce theacid activity of the flux to a mildly activated or non-activated gradein an attempt to minimize the adverse effects of the flux on thecomponents. The problem is exacerbated by the shrinking size ofelectrical components and bonding pads, the growing use of surface mounttechnology, and the increasing demand for flip-chip device bonding.

The post-soldering cleaning step removes any flux residue remaining fromthe precleaning and deoxidation steps. However, as the size ofelectronic components has continued to decrease, the gaps betweenassembled parts and the risk of solidification cavities in solder jointshas made it difficult to do post-soldering cleaning. Inefficient orineffective cleaning can reduce the long term reliability of the entireassembly, causing higher defect levels and higher rework costs. Inaddition, the chemical activity and physical abuse brought about by thepost-soldering cleaning can have its own negative impact. Although somecleaning equipment, newer materials, and refined processes solve some ofthe problems, there may still be undesirable effects and environmentalconcerns.

A fluxless soldering process can replace the precleaning step andvirtually eliminate the post-cleaning step, however, it is stillnecessary to deoxidize the surfaces to insure complete solder reflow andbonding.

Various attempts at fluxless soldering have been made but hightemperatures or pressures are required, or it is necessary to provideexternal energy or stimulation to enable or catalyze the deoxidationprocess.

For example, P. Moskowitz, et al., J. Vac. Sci. Tech. 4, (May/June,1986) describes a dry soldering process for solder reflow and bonding oflead-tin solder using halogen containing gases for the reduction of thesurface oxide to facilitate solder reflow. This process requires the useof a platinum catalyst mesh in a vacuum chamber with a temperature inexcess of 300° C., which can damage delicate electronic components.

IBM Technical Disclosure Bulletin 27 (April, 1985) describes the use ofhalogenated gases in an inert gas carrier at elevated temperatures toproduce a reduction of solder oxides by the reactive gas and to allowsolder reflow. This process requires high temperatures.

P. Moskowitz, et al., J. Vac. Sci. Tech. 3 (May/June, 1985) describes alaser-assisted fluxless soldering technique for solder reflow. However,laser radiation is required to excite an otherwise non-reactive gas inthe presence of a preheated solder surface. This technique requiresdirect access of the laser beam to the solder surface, thus limiting theapplications as well as resulting in a low throughput process.

U.S. Pat. No. 4,921,157 discloses a fluxless soldering process forsemiconductor devices. Solder surface oxides are removed in afluorine-containing plasma assisted process before it is reflowed.

German Patent No. 3,442,538 discloses a method of solderingsemiconductor elements where a semiconductor element having an aluminumlayer is subjected to a fluorine-containing plasma. The treated aluminumsurface is then contacted with a soft solder. Process conditions includetreating the aluminum layer with a fluorine-containing plasma for aboutone hour in a vacuum at a temperature of about 197° C. to 397° C. Thus,high temperatures are required, and throughput is limited.

U.S. Pat. No. 4,646,958 describes a fluxless soldering process usingsilane (SiH₄) at elevated temperatures of about 350° C. to about 375° C.

U.S. Pat. No. 4,821,947 describes a wave soldering process which isperformed in an inert atmosphere at temperatures below 300° C., but areflow process is not described.

U.S. Pat. No. 4,919,729 discloses a soldering paste which may be used ina reducing atmosphere to eliminate flux. However, a reducing atmosphereof hydrogen, heated to approximately 300° C. to 500° C., is required.

U.S. Pat. No. 4,937,006 describes the use of a gas, heated to atemperature sufficient to melt solder, which is directed at the moltensolder at a momentum sufficient to disperse the oxide layer at thesurface of the molten solder. This dispersion allows the oxide layer towet the solder wettable surface.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedsoldering process.

It is another object of the present invention to provide an improvedfluxless soldering process.

It is another object of the present invention to provide a solderingprocess without the need for a post-soldering cleanup.

It is another object of the, present invention to provide improvedsolder flow.

These and other objects of the present invention are accomplished byexposing the solder surface to a vapor including a fluorine compoundwhich deposits on the solder surface without the use of externalstimulation. For example, by exposing the solder surface to a vapor of anoble gas fluorine compound, the surface oxides that inhibit reflow ofthe solder are dispersed or displaced sufficiently so that they do notprevent the wetting of the two surfaces to be joined during the solderreflow step.

The fluxless soldering process of the present invention is believed toproduce fluorine atoms on the solder surface by dissociative adsorptionat room temperature and atmospheric pressure. This is believed to yielda lead-tin alloy solder having fluorine atoms on the exposed surfacethereof, which is believed to displace the oxygen in the tin oxide.

The process conditions of the present invention are flexible. They canbe selected to optimize solder reflow and can be conducted in aconventional processing apparatus. External energy stimulation, hightemperatures, vacuums, and high pressures are unnecessary. Moreover, theprocess gases may be recycled.

DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention and the manner in whichthey are accomplished will be more completely understood with referencesto the detailed description and to the drawings in which:

FIG. 1 shows a deposited solder with a surface oxide coating;

FIG. 2 shows an apparatus for practicing the method of the invention;and

FIG. 3 shows a deposited solder with an adsorbed fluorine coating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more fully with reference tothe accompanying drawings, in which a preferred embodiment of theinvention is shown. This invention can, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, applicants provide this embodimentso that this disclosure will be thorough and complete and will fullyconvey the scope of the invention to those skilled in the art. Likereference numerals refer to like elements throughout. For greaterclarity, the thickness of the layers have been exaggerated, and thecomponents of the processing apparatus have been shown schematically.

FIG. 1 shows a schematic side view representation of a circuit board 5having an electrically conductive metal surface 10 attached to thesurface thereof. Alternatively, surface 10 may be a surface mount boardor another solder bearing surface. For example, it may be a flat surfaceor a tinned lead or other surface to be soldered, and it may include,for example, gold, tin or copper. The solder 20 is deposited on surface10. The solder can be of any appropriate solder material such as tin,lead tin, and lead-tin based alloys. A typical eutectic 63% lead, 37%tin material, having a melting point of 183° C., would be appropriate.It may be a solder preform, solder coated on a surface, solder powder ina paste, or molten solder as in a solder wave reflow tool. The solder isshown in the shape of a ball, which is typical for microelectronicsoldering processes, but it may also conform to any of a wide variety ofsurface configurations for soldering, and it may be applied using one ofseveral techniques, forms, or media.

Surface oxides 25, such as tin oxide, form on the exposed outer surfaceof the solder 20 as a result of being exposed to oxygen or water vapor,including oxygen or water vapor present in the ambient atmosphere. It isthe presence of these surface oxides that prevent or inhibit the solderreflow or the wetting of the surfaces to be solder joined, and musttherefore be at least partially removed to obtain an appropriateelectrical and mechanical connection.

Referring now to FIG. 2, a gas processing apparatus 50 which can be usedto practice the invention is illustrated. The transport mechanismincludes an endless conveyor belt 52 that is driven by and passes overrollers 54 at opposite ends to advance the conveyor belt through theprocessing chambers in the direction of arrow 56. Support 58 providesvertical positioning of the conveyor belt as it passes through theprocessing chambers.

The processing apparatus includes three chambers 60, 70 and 80, arrangedin sequence. They are illustrated being adjacent one another so that anitem to be processed, such as a circuit board 5, will move sequentiallythrough the three illustrated chambers.

The first chamber, purge chamber 60, includes walls defining a cavityabove the circuit board 5 to be processed. The upper portion includes amechanical structure or hood 63. The lower portion is defined byconventional air or safe gas curtains 64, e.g., dry nitrogen, to isolatethe interior of chamber 60 from the ambient conditions and adjacentchambers, and to contain any gases introduced into the chamber.Alternatively, a load lock may be used. The conveyor belt 52 defines thelower portion of the chamber. The upper portion of the chamber, aircurtains, and conveying mechanism are well known to those in theindustry. One of the benefits of the present invention is that it may bepracticed at room temperature and atmospheric pressure, greatlysimplifying the construction of the processing apparatus and minimizingits cost. In the alternative, however, and as an added benefit of theinvention, it may be practiced in a variety of environments and under avariety of conditions that will best fit with the various stations in anelectronics manufacturing process requiring soldering. The purge chamberincludes inlet 61 and exhaust 62 for the purge gas, such as argon ornitrogen, that is used to purify the ambient atmosphere for the circuitboard and eliminate contaminants that would otherwise interfere with thefluorination process in the succeeding chamber.

Adjacent the purge chamber 60 is the processing chamber 70. Itsconstruction is similar to that of the purge chamber and is separatedtherefrom by an air curtain 64 so that the circuit board may moveunimpeded from the purge chamber to the processing chamber. A gas inlet71 and exhaust 72 allow the processing gas to surround the soldercarrying circuit board. The process gas is a weakly internally bondedfluorine gas, typically XeF₂, in an argon flow to carry the XeF₂ fromits source to the solder surface. The XeF₂ source 74 may be a solidmaterial, which is volatile and has a very low surface pressure, causingit to give off the desired fluorine compound enriched vapors. An inertcarrier, such as an argon source 75, produces a gas flow which includesthe XeF₂ as it passes through the processing gas inlet 71 to a nozzle 73that will cause the reactive gas blend 76 to suffuse the solder surfaceson the circuit board. The exhaust port 72 may be used to collect theprocessing gas, which may be recycled.

Adjacent the processing chamber is an exhaust chamber 80 having aconstruction similar to the purge chamber 60. It is separated from theprocessing chamber 70 by a wall and an air curtain 64 which permits thecircuit board 5 to freely move from the processing chamber into theexhaust chamber. An inlet port 81 and an exhaust port 82 for the purgegas clarifies the atmosphere surrounding the circuit board and removesany last traces of the XeF₂ or other noble fluorine compound used in theprocess.

The circuit board exits the processing apparatus through the air curtain64, and may proceed to a subsequent processing station.

In operation, the solder or other surface on the circuit board 5 isdirectly fluorinated by vapors of a thermally dissociated sourcemolecule. XeF₂, which is a solid material whose vapor pressure isapproximately 6 Torr at 70° C., is the preferred fluorine source, but itis believed that any noble fluorine compound may used. A XeF₂ flow maytake place at atmospheric pressure and without external thermalexcitation or other energy sources or stimulation such as plasmas orlasers. XeF₂ is moisture sensitive and is well known to dissociativelyadsorb on metallic surfaces. Under atmospheric pressure (760 Torr) thesolid material vaporizes.

It is further believed that the XeF₂ dissociatively adsorbs on themetallic surface to replace the tin oxide with tin fluoride. Althoughthe exact mechanism of this process is not understood, it is believedthat the surface fluorine in the compound dissolves into the solder, orbreaks up the surface oxide into colloidal-type particles, or replacesthe tin oxide on the surface of the solder. Any other material whichproduces fluorine atoms on a metal surface by dissociative adsorption onthe surface may also be used. It is believed that the followingstoichiometric process occurs on a solder surface: ##STR1## The equationreflects that the XeF₂ reacts with the SnO₂ (tin oxide) on the soldersurface to chemically convert it and yield free xenon and fluorine. Thefluorine reacts with the tin oxide on the solder surface and is believedto ultimately yield free oxygen and tin-fluoride or it may yield waterand tin oxyfluoride. In typical operation, the fluorination process isperformed until the surface oxides have been sufficiently removed fromthe solder surface or chemically converted to facilitate reflow. Undertypical circumstances the circuit board to be suffused with the noblefluorine gas for approximately 20 to 40 minutes in a vacuum partialpressure at 300 millitorr, or at atmospheric pressure with less than 10percent of the process gas being XeF₂.

Fluorination is believed to result in the conversion of at least some ofthe oxide layer on the surface of the solder to tin fluoride or tinoxyfluoride, or may result in removal of at least some of the oxidelayer from the surface of the solder and formation of a tin fluoridecompound in its place, as shown schematically on the solder deposit inFIG. 3. It is similar to the solder deposit illustrated in FIG. 1, withthe surface oxides 25 absent or replaced by a surface fluorine compound40, consistent with the beliefs expressed regarding the mechanism ofthis process. The fluorinated solder is then reflowed to form a newsolder bump or to reflow and join to a second surface. The secondsurface may be another component or solder compound and may be conductedunder typical conditions used with conventional wet flux methods.

Because the process takes place at atmospheric pressure, a vacuumchamber is not necessary to practice this method, but one may be used.Moreover, since external temperatures or other energy stimulation is notnecessary, such as a plasma or laser beam, the adverse effects of hightemperature and complicated machinery may be avoided. A simple beltdriven transport system may be used to move the solder bearing partsthrough a chamber containing a vapor phase noble fluorine gas. Thechamber may or may not include a nozzle 73 to direct the gas flow. And,because the amount of fluorine needed is small, the concentration of thenoble fluorine gas may be low. This transport system may be implementedin a system similar to a conventional solder reflow machine.

It is believed that this process is compatible with other pretreatmentprocesses, such as oxygen plasma treatments. The oxygen plasma treatmentwill remove, by oxidation, any organic residue from the surface andeliminates any need for presolder cleaning.

In the drawings and specification there have been disclosed typicalpreferred embodiments and, although specific terms are employed, theyare used in a generic and descriptive sense only and not for purposes oflimitation. The scope of the invention is set forth within the followingclaims.

That which is claimed is:
 1. A method of soldering without the need forfluxing agents, comprising the steps of:depositing solder on a firstsurface; exposing the deposited solder to a vapor including a fluorinecompound which suffuses the solder surface without the use of externalstimulation; and reflowing said solder.
 2. The method of claim 1 whereinthe step of exposing the deposited solder comprises exposing thedeposited solder to a vapor of a material which produces a fluorinecompound.
 3. The method of claim 1 wherein the step of exposing thedeposited solder comprises exposing the deposited solder to a vapor of amaterial which includes fluorine at room temperature and atmosphericpressure.
 4. The method of claim 1 wherein the step of exposing thedeposited solder comprises exposing the deposited solder to a vapor froma noble gas fluorine compound.
 5. The method of claim 1 wherein the stepof exposing the deposited solder comprises exposing the deposited solderto a vapor that produces fluorine on the solder surface by dissociativeadsorption.
 6. The method of claim 1 wherein the step of exposing thedeposited solder comprises exposing the deposited solder to the vapor ofXeF₂.
 7. The method of claim 1 wherein the step of exposing thedeposited solder occurs in a vacuum.
 8. The method of claim 1 whereinthe step of exposing the deposited solder occurs for approximately 20 to40 minutes.
 9. A method of chemically converting oxides on a surface andfor promoting the wetting of a surface to be joined together bysoldering comprising the step of exposing the surface to a vaporincluding a fluorine compound which suffuses the surface without the useof external stimulation.
 10. The method of claim 9 wherein the step ofexposing the surface comprises exposing the surface to a vapor thatproduces fluorine on the surface by dissociative adsorption.
 11. Themethod of claim 9 wherein the step of exposing the surface comprisesexposing the surface to a vapor of a material which includes fluorine atroom temperature.
 12. The method of claim 9 wherein the step of exposingthe surface comprises exposing the surface to a vapor of a materialwhich includes fluorine at atmospheric pressure.
 13. The method of claim9 wherein the step of exposing the surface comprises exposing thesurface to a vapor from a noble gas fluorine compound.
 14. The method ofclaim 9 wherein the step of exposing the surface comprises exposing thesurface to the vapor of XeF₂.
 15. The method of claim 9 wherein the stepof exposing the surface comprises exposing the surface of an electricalconductor.
 16. The method of claim 9 wherein the step of exposing thesurface comprises exposing the surface of a solder deposit.
 17. A methodof soldering without the need of fluxing agents, comprising the stepsofdepositing solder on a first surface to be soldered; exposing thedeposited solder to a vapor of a noble gas fluoride compound untilsufficient fluorine forms on the exposed surface of said solder topermit wetting to occur; and reflowing said solder.
 18. The method ofclaim 17 wherein the step of exposing the deposited solder comprisesexposing the deposited solder to a vapor of a noble gas fluoridecompound at room temperature and atmospheric pressure.
 19. The method ofclaim 17 wherein the step of exposing the solder surface comprisesexposing the solder surface to the vapor of XeF₂.
 20. The method ofclaim 17 wherein the step of exposing the deposited solder comprisesexposing the deposited solder until at least some of the surface oxideson the deposited solder are chemically converted.
 21. The method ofclaim 17 wherein the step of exposing the deposited solder comprisesexposing the deposited solder until sufficient fluorine forms on theexposed surface of said solder by dissociative adsorption.
 22. Themethod of claim 17 wherein the step of exposing the deposited solderfurther includes exposing the deposited solder at room temperature. 23.The method of claim 17 wherein the step of exposing the deposited solderfurther includes exposing the deposited solder at atmospheric pressure.24. A method of exposing a surface to be soldered comprising exposingthe surface to the vapor from XeF₂.
 25. The method of claim 24 whereinthe step of exposing the surface to the vapor from XeF₂ furthercomprises exposing the surface to the vapor from XeF₂ in an inertcarrier gas flow.
 26. The method of claim 24 wherein the step ofexposing the surface further comprises exposing the surface at roomtemperature.
 27. The method of claim 24 wherein the step of exposing thesurface further comprises exposing the surface at atmospheric pressure.28. The method of claim 24 wherein the step of exposing the surfacefurther comprises exposing the surface without the use of externalstimulation.